This is shiso, a plant in the mint family. The stems and undersides of the leaves are purple, and the plant resembles coleus. (There is also a green-stemmed variety, and a frillier type.) Shiso, a form of perilla, originated in East or Southeast Asia, and is mentioned in a text from around 500 CE. The purple form, called akajiso, is used for coloring pickled plum (umeboshi) and as a distinctive flavor reminiscent of mint and cinnamon in a variety of dishes. The green type is probably better as a fresh green, but the purple kind adds an ornamental element to the garden.